This invention pertains to the field of non-invasive in vivo measurement of blood components such as glucose, hemoglobin, and bilirubin.
The measurement of the concentration of blood components such as hemoglobin and glucose has required the drawing of a blood sample for in vitro analysis. The need to draw blood for analysis is undesirable for several reasons, including discomfort to the patient, the time required of medical personnel to draw and handle the samples, and the potential risk of spread of disease through punctures of the skin. Repeated drawing of blood samples is especially undesirable in infants. Many diabetics must test their blood up to six times a day to monitor their blood glucose levels. It would thus be desirable to be able to obtain fast and reliable estimates of the concentration of blood components in blood, such as hemoglobin and glucose, through a simple and non-invasive technique. Prior efforts have involved an examination of blood in the skin or extremities, such as fingers and ear lobes, or in observable surface blood vessels, but these efforts have had limited practical success due to the presence of tissue components that interfere with accurate reading of only the concentration of blood components.
There are approximately four million newborns in the United States alone each year. About 50% of newborns are clinically jaundiced from elevated bilirubin levels. If the serum bilirubin reaches very high levels during the post-natal period, kernicterus, neural damage resulting from sustained high levels of serum bilirubin, may occur. Frequent monitoring of serum bilirubin is critical to the care of these infants. Of the newborns that have recognizable jaundice during the first 5 days of life, 1.7 million receive at least one blood test for bilirubin. Of those tested, about 700,000 undergo phototherapy treatment; these infants receive an estimated two to three additional blood tests. Presently, blood is drawn through the heel of the neonate, resulting in occasional infections and other complications. Other drawbacks to this process are its high cost and the delay in lab results reaching the physician. Recently introduced non-invasive devices for measuring bilirubin do not provide the accuracy level required to diagnose or treat elevated serum bilirubin levels, rendering them virtually useless in practice.
The present invention combines the accuracy of in vitro laboratory testing of blood components and the advantages of rapidly-repeatable non-invasive technology. The invention utilizes a hand-held or stationary instrument for retinal imaging that allows non-invasive measurement of certain blood components in the retinal blood vessels. Illuminating light of selected wavelengths in the visible or infrared range is projected into the eye onto the fundus, and the light reflected back and out (e.g., through the pupil) is detected and analyzed, preferably using the area of the optic disk for analyzing the retinal vessels overlaying the optic disk for most blood components to be measured. Specific wavelengths of illuminating light may be chosen for each blood component to be analyzed depending on the spectral characteristics of the particular substance being analyzed. The reflected image from the retina is utilized to measure blood components, such as hemoglobin, glucose and bilirubin.
The utilization of the retina as a site for obtaining blood component data has several advantages, including the ease of visualizing the data because of the natural window provided by the eye. The reflected light from the fundus at visually significant wavelengths is much less scattered than light reflected from the skin or mucous membranes since the eye is naturally immune to scatter. The retina creates a uniform background for imaging, and the optical devices and techniques required for obtaining retinal images have been extensively developed and studied because of the need for ophthalmologists to image the retina for diagnosis of disease states. In addition, the blood flow to the retina is very even and repeatable even across a number of disease states. For example, although patients in shock have reduced blood flow to the skin and mucous membranes, allowing false data to be obtained with current technology that examines the skin and mucous membranes, the body maintains even blood flow to the retina except in states of extremely low blood pressure. Furthermore, in the present invention, there is no physical contact required between the device and the skin or mucous membranes, thereby eliminating the potential for transmission of infectious agents associated with devices that require patient contact. The device may be rested upon the orbit of the patient for ease of use and, if desired, a disposable plastic cover may be used to further minimize the risk of transmission of infectious agents. Such a cover may be transparent and fully cover the device, or not, depending upon requirements of the imaging system and the need to prevent incidental contact with the device.
In accordance with the invention, a hand-held or stationary instrument for retinal imaging may be used to obtain non-invasive measurement of photoreactive analytes, an example of which is serum bilirubin. Illuminating light of selected wavelengths in the visible range is projected into the eye onto the fundus. Specific wavelengths of illuminating light are chosen so that serum bilirubin can be measured. Analysis of the reflected image from the fundus is utilized to measure bilirubin. Although measurement of the reflected light from the vessels overlying the optic disk is preferred, in accordance with the invention, it is also typically possible to obtain bilirubin measurements from light reflected from the fundus generally.
During disease states when the serum bilirubin levels are above normal (e.g., newborn jaundice), bilirubin is extruded from the choroid into the nerve layer of the retina. During newborn jaundice, this nerve layer stains yellow from the elevated bilirubin levels. This yellow color is directly proportional to the elevated serum bilirubin levels and changes rapidly with changes in serum bilirubin. The bilirubin molecule exhibits peak absorption of light at 470 nm. However, when exposed to light at or near this wavelength, the molecule breaks down into optically inactive molecules. The intact bilirubin molecule reflects light at and near a wavelength of approximately 550 nm (yellow light), and is not affected by this light. In the present invention, the retina of the patient""s eye may first be imaged with light that does not break down bilirubin, e.g., light at a wavelength of 550 nm with little or no light at 470 nm. The intensity of the reflected light at or near the maximum reflection wavelength of 550 nm is detected. Then the retina is imaged again using light that breaks down bilirubin, e.g., using light at 470 nm followed by or combined with light at 550 nm, which is projected into the eye. The reflected light at 550 nm that is passed out through the pupil is detected to image the retina a second time. With the addition of light at 470 nm, the bilirubin molecule is rendered optically inactive and will no longer reflect at 550 nm. The difference in the reflected image intensity at 550 nm from the first image to the second image is a function of the bilirubin concentration. A neural network or other processing technique may be used to analyze the two data sets of the images captured by the retinal camera.
Further objects, features and advantages will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.